1. Technical Field
The present disclosure relates to a technique of coding or decoding, using vector quantization, an acoustic signal such as a voice signal or a music sound signal.
2. Description of the Related Art
It is known to use vector quantization to code or decode an acoustic signal such as a voice signal or a music sound signal. A specific example of this method is algebraic vector quantization (AVQ) in which quantization is performed on pulses within a predetermined quantization bit rate as disclosed, for example, in Stephane Ragot, Bruno Bessette, Roch Lefebvre, “Low-complexity Multi-rate Lattice Vector Quantization With Application To Wideband TCX Speech Coding at 32 kbit/s”, ICASSP 2004. In this technique, an input signal is converted by MDCT (Modified Discrete Cosine Transform) or the like to a frequency-domain signal (spectrum) in units of frames each including a predetermined number of samples, and the resultant signal is divided into a plurality of a subbands. In vector quantization employed in this technique, bits for quantization are assigned only to a part of spectrum of each subband, and “0” is assigned to the remaining part of the spectrum.
However, in the vector quantization, if a situation occurs in which a predetermined number of quantization bits is not sufficient to quantize all frequency components, an perceptually important spectral component may be lost, without being quantized, from some temporally successive frames, which may result in audible distortion. This phenomenon is known as a spectrum hole.
To handle the above situation, International Publication No. 2012/005209 discloses a coding method in which, first, a quantized normalized value is determined by quantizing a normalized value which is a representative value of a predetermined number of samples, and then a normalized value quantization index corresponding to the quantized normalized value is determined. In a case where when each sample value is subtracted by a value corresponding to the quantized normalized value, if the resultant subtracted value is positive and the sample value is also positive, then the subtracted value is employed as a value to be subjected to quantization corresponding to the sample, but if the subtracted value is positive and the sample value is negative, then the sing of the subtracted value is inverted and the resultant value is employed as the value to be subjected to quantization corresponding to the sample. The value to be subjected to quantization is then vector-quantized thereby determining the vector quantization index. The resultant vector quantization index is output. Using this method, major components including samples which would not be subjected to the vector quantization based on the AVQ method or the like are selected from all frequency components and the selected major components are intentionally quantized. This allows it to prevent an occurrence of a spectrum hole in the major component of the decoded signal.
International Publication No. 2011/086900 discloses a technique of correcting spectral data before it is converted into a lattice vector. For example the correction is performed such that values other than values of perceptually important samples are set to zero, thereby improving quality of a decoded signal. This technique can be performed at a low bit rate with a small amount of calculation.
Another improvement in the AVQ method may be found, for example, in International Publication No, 2011/132368. A description of other related techniques may be found, for example, in Recommendation ITU-T G.718, SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS, Digital terminal equipments—Coding of voice and audio signals, Frame error robust narrow-band and wideband embedded variable bit-rate coding of speech and audio from 8-32 kbit/s.